1. Technical Field
This invention relates to an EL device comprising at least an electrically insulating substrate and a structure including a patterned electrode layer on the substrate and a dielectric layer, a light emitting layer and a transparent electrode layer stacked on the electrode layer.
2. Background Art
EL devices are on commercial use as backlight in liquid crystal displays (LCD) and watches.
The EL devices utilize the phenomenon that a material emits light upon application of an electric field, known as electroluminescent phenomenon.
The EL devices include dispersion type EL devices of the structure that a dispersion of powder luminescent material or organic material in enamel is sandwiched between electrode layers, and EL devices in which a light emitting thin-film sandwiched between two electrode layers and two insulating thin films is formed on an electrically insulating substrate. For each type, the drive modes include dc voltage drive mode and ac voltage drive mode. The dispersion type EL devices are known from the past and have the advantage of easy manufacture, but their use is limited because of a low luminance and a short lifetime. On the other hand, the EL devices are currently on widespread use on account of a high luminance and a long lifetime.
FIG. 2 shows the structure of a dual insulated thin-film EL device as a typical prior art EL device. This thin-film EL device includes a transparent substrate 21 of a blue sheet glass customarily used in liquid crystal displays and plasma display panels (PDP), a transparent electrode layer 22 formed from ITO or the like in a predetermined stripe pattern to a thickness of about 0.2 to 1 xcexcm, a thin-film transparent first insulator layer 23, a light emitting layer 24 having a thickness of about 0.2 to 1 xcexcm, and a thin-film transparent second insulator layer 25, all stacked on the substrate 21, and a metal electrode layer 26 of Al thin film or the like which is patterned into stripes extending perpendicular to the transparent electrode layer 22. A voltage is selectively applied to a specific light-emitting material selected in the matrix formed by the transparent electrode layer 22 and the metal electrode layer 26, whereby the light-emitting material in the selected pixel emits light which comes out from the substrate 21 side. The thin-film transparent insulator layers 23, 25 have a function of restricting the current flow through the light emitting layer 24 in order to restrain breakdown of the thin-film EL device and act so as to provide stable light-emitting characteristics. Thus thin-film EL devices of this structure are on widespread commercial use.
The thin-film transparent insulator layers 23, 25 mentioned above are generally transparent dielectric thin-films of Y2O3, Ta2O5, Al3N4, BaTiO3, etc. deposited to a thickness of about 0.1 to 1 xcexcm by sputtering and evaporation techniques.
Among light emitting materials, Mn-doped ZnS which emits yellowish orange light has been often used from the standpoints of ease of deposition and light emitting characteristics. The use of light emitting materials which emit light in the primaries of red, green and blue is essential to manufacture color displays. Known as the light emitting materials are Ce-doped SrS and Tm-doped ZnS for blue light emission, Sm-doped ZnS and Eu-doped CaS for red light emission, and Tb-doped ZnS and Ce-doped CaS for green light emission.
Also, monthly magazine Display, April 1998, Tanaka, xe2x80x9cTechnical Trend of Advanced Displays,xe2x80x9d pp. 1-10, sets forth a variety of light emitting materials, for example, ZnS and Mn/CdSSe as the red light emitting material, ZnS:TbOF and ZnS:Tb as the green light emitting material, and SrS:Cr, (SrS:Ce/ZnS)n, Ca2Ga2S4: Ce, and Sr2Ga2S4: Ce as the blue light emitting material. Also disclosed are light emitting materials capable of emitting white light such as SrS:Ce/ZnS:Mn.
It is further disclosed in International Display Workshop (IDW), ""97, X. Wu, xe2x80x9cMulticolor Thin-Film Ceramic Hybrid EL Displays,xe2x80x9d pp. 593-596, that among the aforementioned materials, SrS:Ce is used in thin-film EL devices having a blue light emitting layer. It is also described in this article that when a light emitting layer of SrS:Ce is formed, deposition in a H2S atmosphere by an electron beam evaporation technique results in a light emitting layer of high purity.
Nevertheless, for these thin-film EL devices, a structural problem remains still unsolved. Specifically, since the insulator layer is formed of a thin film, it is difficult to manufacture displays having large surface areas while completely eliminating steps at the edge of a transparent electrode pattern and avoiding defects in the thin-film insulator introduced by debris or the like in the manufacturing process. This leaves a problem that the light emitting layer fails on account of a local drop of dielectric strength. Such defectives impose a fatal problem to display devices. This creates a substantial barrier against the widespread commercial application of thin-film EL devices as large-area displays, in contrast to liquid crystal displays and plasma displays.
To solve the problem of defects in the thin-film insulator, JP-B 7-44072 discloses an EL device which uses an electrically insulating ceramic substrate as the substrate and a thick-film dielectric material instead of the thin-film insulator underlying the light emitting layer. Since the EL device of the above patent is constructed such that light emitted by the light emitting layer is extracted from the upper side remote from the substrate as opposed to prior art thin-film EL devices, a transparent electrode layer is formed on the upper side.
Further, in this EL device, the thick-film dielectric layer is formed to a thickness of several tens to several hundreds of microns, which is several hundred to several thousand folds of the thickness of the thin-film insulator layer. This minimizes the potential of breakdown which is otherwise caused by steps of electrodes and pinholes formed by debris in the manufacturing process, offering the advantages of high reliability and high yields during manufacture. Meanwhile, the use of such a thick-film dielectric layer entails a problem that the effective voltage applied across the light emitting layer drops. For example, the above-referred JP-B 7-44072 overcomes this problem by using a complex perovskite high-permittivity material containing lead in the dielectric layer.
However, the light emitting layer formed on the thick-film dielectric layer has a thickness of several hundreds of nanometers which is merely about 1/100 of that of the thick-film dielectric layer. This requires that the thick-film dielectric layer on the surface be smooth at a level below the thickness of the light emitting layer although a conventional thick-film procedure is difficult to form a dielectric layer having a fully smooth surface.
Specifically, the thick-film dielectric layer is essentially constructed of a ceramic material obtained using a powder raw material. Then intense sintering generally brings about a volume contraction of about 30 to 40%. Unfortunately, although customary ceramics consolidate through three-dimensional volume contraction upon sintering, thick-film ceramics formed on substrates cannot contract in the in-plane directions of the substrate under restraint by the substrate, and is allowed for only one-dimensional volume contraction in the thickness direction. For this reason, sintering of the thick-film dielectric layer proceeds insufficiently, resulting in an essentially porous body. Moreover, since the surface roughness of the thick-film is not reduced below the crystal grain size of the polycrystalline sintered body, its surface have asperities greater than the submicron size.
In the presence of the surface defects, porosity and asperities of the dielectric layer as mentioned above, the light emitting layer that is formed thereon by vapor phase deposition techniques such as evaporation and sputtering conforms to the underlying surface profile and thus cannot be uniform. It is then difficult to effectively apply an electric field across light emitting layer regions formed on uneven areas of the substrate, resulting in a reduction of effective luminous area. On account of local unevenness of film thickness, the light emitting layer undergoes partial breakdown, resulting in a lowering of emission luminance. Moreover, since the film thickness has large local variations, the strength of the electric field applied across the light emitting layer has large local variations as well, failing to provide a definite emission voltage threshold.
To solve these and other problems, for example, JP-A 7-50197 discloses a procedure of improving surface smoothness by stacking on a thick-film dielectric of lead niobate a high-permittivity layer of lead titanate zirconate or the like to be formed by the sol-gel technique.
The use of ceramic high-permittivity dielectric thick-films in this way makes it possible to avoid steps at the pattern edge of lower electrode layer, and defects introduced in thin-film insulator by debris during the manufacturing process, thereby solving the problem that the light emitting layer can break down on account of local drops of dielectric strength.
However, EL devices using such prior art ceramic high-permittivity thick-films have to use lead base dielectric layers as the high-permittivity thick-film layer in order to acquire such characteristics as low-temperature sintering ability, high permittivity and high dielectric strength. Unfortunately, where lead base dielectric materials are used as the dielectric layer material, the light emitting layer formed on the dielectric layer can react with lead components in the dielectric layer, resulting in a lowering of initial emission luminance, luminance variations, and changes with time of emission luminance, all undesirable on practical use.
An object of the invention is to provide an EL device which has solved the lowering, variations, and changes with time of emission luminance of EL devices using lead base dielectric materials, and affords high display quality without increasing the cost.
This and other objects are attained by the construction defined below as (1) to (8).
(1) An EL device comprising at least an electrically insulating substrate and a structure including an electrode layer, a dielectric layer, a light emitting layer and a transparent electrode layer stacked on the substrate in the described order, wherein
said dielectric layer is a laminate including a first thick-film ceramic high-permittivity dielectric layer whose composition contains at least lead, a second high-permittivity layer whose composition contains at least lead, and a third high-permittivity layer whose composition is free of at least lead.
(2) The EL device of (1) wherein said third high-permittivity layer is formed of a perovskite structure dielectric material whose composition is free of at least lead.
(3) The EL device of (1) or (2) wherein said second and third high-permittivity layers are formed by a solution coating-and-firing technique.
(4) The EL device of (1) or (2) wherein said second high-permittivity layer is formed by a solution coating-and-firing technique, and said third high-permittivity layer is formed by a sputtering technique.
(5) The EL device of any one of (1) to (4) wherein said third high-permittivity layer has a thickness of more than 0.2 xcexcm.
(6) An EL device comprising at least an electrically insulating substrate and a structure including an electrode layer, a dielectric layer, a light emitting layer and a transparent electrode layer stacked on the substrate in the described order, wherein
said dielectric layer is a laminate including a thick-film ceramic high-permittivity dielectric layer whose composition contains at least lead and a second high-permittivity layer formed of a dielectric material whose composition is free of at least lead.
(7) The EL device of (6) wherein said second high-permittivity layer is formed of a perovskite structure dielectric material whose composition is free of at least lead.
(8) The EL device of (6) or (7) wherein said second high-permittivity layer is formed by a solution coating-and-firing technique.